PCS Control Modes Explained: PQ, V/F and VSG in Battery Storage

A battery cabinet stores energy; its power conversion system (PCS) determines how that energy interacts with the AC network. PQ, V/F and VSG are common control terms in storage quotations, but they describe different control objectives. Understanding them helps factory owners and EPCs specify the operating behaviour they need before selecting a battery capacity or cabinet count.

YINTU is an energy storage solution provider. We work with capable manufacturing partners to match equipment, operating requirements and project interfaces. Our role includes configuration selection, technical clarification and supply coordination; the local EPC, installer and other project partners undertake the engineering and services agreed in the project scope.

The three modes at a glance

These labels are a useful starting point, not a complete performance specification. A PCS may offer several modes, and manufacturers use different names and implementations. Verify the exact supplied model, firmware and operating-mode matrix.

PQ control: dispatching active and reactive power

P is active power, measured in kW, which transfers useful energy to or from the battery. Q is reactive power, measured in kvar, which influences power factor and voltage conditions. An EMS might request battery discharge at a specified kW while the PCS follows a separately defined reactive-power command. Charging and discharging sign conventions must be agreed with the controller supplier.

In the conventional grid-following implementation, the PCS synchronizes to an existing AC waveform, commonly using a phase-locked loop. The reference can come from the utility or another suitable source in a microgrid. PQ operation alone does not establish an independent energized bus after that reference disappears. However, grid-following equipment can still provide configured frequency or voltage support by adjusting P and Q; it is inaccurate to say it offers no support at all.

Apparent power matters. If a PCS must deliver both P and Q, its kVA rating and current limits constrain the combination. A high active-power request may leave less reactive-power headroom. Ask for the P-Q capability curve at the required voltage and temperature, rather than assuming every command can be met simultaneously.

V/F control: supplying a local AC reference

In voltage-and-frequency control, the inverter regulates the local voltage waveform and frequency. This is a voltage-forming function and can support an islanded bus when the battery, auxiliaries, protection and switching arrangement permit it. As loads change, the PCS supplies the required power within its current, energy and thermal limits; it cannot supply unlimited motor-starting current or operate indefinitely without recharge.

A simple fixed V/F controller should not be connected in parallel with another stiff voltage source without the synchronization and sharing controls designed for that arrangement. Multi-inverter operation may use a master controller, droop or other coordination methods. V/F can also be part of a black-start design, so black start is not exclusive to VSG.

VSG control: emulating machine behaviour

A virtual synchronous generator uses control software to emulate selected dynamics of a synchronous machine. Configured inertia and damping shape how its internal frequency and active-power response evolve after a disturbance. Voltage and reactive-power control complete the interaction with the AC network. The response still depends on available battery energy, power headroom and converter current limits.

VSG is one approach used for grid-forming control. Droop control, virtual oscillator control and other methods also exist. A VSG label does not prove a universal transfer time, guaranteed fault current, automatic SOC equalization or compatibility with any other inverter. Droop-based primary power sharing can reduce dependence on fast communications, while secondary control and energy management may still require communication and coordinated settings.

Match the mode to a real C&I operating objective

• Factory peak management: specify the grid-meter target, EMS dispatch, recharge opportunities and reserve SOC. PQ dispatch is often the relevant operating objective.
• Essential circuits during an outage: define the isolated bus, starting loads and permitted interruption. Confirm the voltage-forming function and the complete transfer sequence.
• Solar-diesel microgrid: agree which source forms the bus, how PV is curtailed when batteries are full, and how generators and multiple PCS units share power.
• Weak-grid support: request the actual grid-forming control characteristics, validated models and site-specific stability studies. A generic brochure term is insufficient.

For a compact AC-connected system, review YINTU’s 125 kW / 261 kWh liquid-cooled C&I storage cabinet. Its listing identifies integrated PCS, BMS and EMS equipment. For larger solar-storage schemes, explore the 1 MW / 2089 kWh containerized hybrid ESS with direct PV input. These are equipment starting points; the product size and the word “hybrid” do not establish a particular VSG or grid-forming capability.

What to request before ordering

• A control-mode matrix covering grid-connected operation, intentional islanding, restart and reconnection.
• Continuous kW/kVA ratings, P-Q capability, overload duration and current-limiting behaviour.
• The supported voltage, frequency, phase arrangement, earthing method and load imbalance limits.
• A single-line diagram identifying the point of common coupling, isolation devices and critical-load bus.
• Factory and site acceptance tests for load steps, source loss, PV changes, communication failure and restoration.

Frequently asked questions

Is VSG always better than PQ?

The appropriate control follows the service required. A grid-connected tariff-management project may not need advanced islanded operation. A microgrid with multiple sources needs a different specification. Compare the tested behaviour and total project scope.

Does V/F mean motor-drive V/Hz control?

Here V/F means PCS AC-bus voltage and frequency regulation. The same abbreviation is used for volts-per-hertz motor control, so state the intended meaning in technical documents.

Plan your storage solution with YINTU

Send YINTU the site voltage and frequency, load profile, existing PV and generator details, operating modes and acceptable interruption. We coordinate a suitable equipment proposal and manufacturer clarification with your project partners.

Discuss your project requirements with YINTU.

Technical reading

• Schneider Electric: microgrid inverter control-mode definitions
• Sandia National Laboratories: grid-forming and grid-following droop-response comparison